Bending equipment is known for bending lamellar, thermoplastic workpieces which comprise at the center of the machine frame a stationary clamping means followed--as seen in the direction of advance of the workpiece--by a pivotably suspended, strip-like bending cheek. A heater and a holder are mounted behind the clamping means on a frame displaceable in and opposite the direction of advance. The heater consists of two superposed heater blades behind which is mounted the holder, which assumes the form of a clamping strip. A stop strip is present on the other side of the bending cheek and rests displaceable on two side rails.
To carry out bending, first the stop strip and the frame with the heater and holder are arranged symmetrically to the front edge of the bending cheek and locked in place. Thereupon the workpiece is inserted in such a way that it comes to rest between the clamping strips of the holder and the two heater blades. At its front edge the workpiece abuts the pertinent zone of the bending cheek. Thereupon the two heater blades are so moved together that they shall rest against both sides of the workpiece which they heat while forming a bending zone, whereby the workpiece is plastically deformable in the bending zone. The workpiece in this process is held in place both by the clamping means and the holder.
Once the bending zone has been sufficiently heated, the clamping means and the holder are deactivated, that is, the workpiece is released. Then it is manually moved forward toward the stop strip until it comes to rest against it. Thereupon the clamping strip is reactivated, that is, the workpiece is braced broadside in front of the bending cheek and clamped in place. Next the bending cheek is pivoted upward by the desired bending angle, whereby the part of the workpiece in front of the clamping means is bent up at the bending zone. Simultaneously the next bending zone is formed by the heater blades again being made to rest against that part of the workpiece. After termination of bending, the clamping means and holder are released again and the workpiece is advanced as far as the stop strip, so that the heated bending zone comes to rest against the front edge of the bending cheek. Another bending procedure follows. Depending on the bending angle a cross-sectionally polygonal, preferably square pipe segment can be formed.
A further development of this machine is described in the German patent 36 37 436. In this bending machine the clamping means is displaceable in and opposite the direction of advance of the workpiece. It no longer serves only to lock in place by means of the bending cheek the workpiece during bending but also now assumes the holder function of the previously known bending machine, namely the locking in place of the workpiece while the bending zone is formed by the heater. For that purpose the clamping means when open is displaceable from a position near the bending cheek into a position away from the bending cheek as seen in relation to the bending zone and is moved into the clamping position. After the bending zone has been formed, the clamping means is displaced again toward the bending cheek and in the process carries the workpiece with the heated bending zone as far as the bending cheek. In this bending machine the holder merely meets the function of locking the workpiece when the clamping means is moved to the deactivated state, that is the open state, away from the bending cheek. In view of this design of the bending machine, it is possible by means of suitable controls to fully automate bending so that upon insertion of the workpiece and following setting the distances between two bending zones, further intervention shall not be required. In a special design of this bending machine, the heater is mounted on the clamping means, whereby both are always displaced jointly.
These known bending machines are only suited to bend workpieces consisting purely of a thermoplastic. Recently however thermoplastic composites have assumed increasing significance. These are especially glass-, carbon- or aramid-fiber reinforced thermoplastics, both with short fibers and with filaments. In the latter case the fibers may be present in random form as matting or as fiber strands or bands processed into fabrics or superposed, crossing filaments. In particular as regards more recent plastics with high processing temperatures, for instance PEEK (polyetheresterketone), PPS (polyphenylene sulfide) or PEI (polyetherimide), fiber reinforcement is frequently used to produce plastics parts of high strength, with applications foremost in the aircraft industry. In these materials the thermoplastic forms the matrix for the fiber reinforcement. The fiber reinforcement is imbedded--especially as regards the above cited materials--with heating and high pressure, this pressure being applied either in a mold or by means of rolls.
Moreover sandwich panels with thermoplastic cover layers and an inserted support core are also known as thermoplastic composites.
Substantial problems arise when such thermoplastic composites are subjected to bending. Where fiber-reinforced thermoplastics are concerned, bending is opposed by the fibers being unable to change their length. Accordingly when bending takes place, there takes place delamination at the outside of the bending zone, that is the bond between the plastic matrix and the fibers is lifted, or fibers will rupture. On the inside the fibers are upset and then project from the matrix material. Either condition entails an appreciable loss of strength in the bending zone of the workpiece. All attempts at palliation so far have led to unsatisfactory results or else entailed excessive expense.